The invention relates, generally, to cryogen storage dewars and, more particularly, to a level sensor and flush system for such devices.
Cryobiological storage dewars are used to store heat sensitive products such as biological specimens at temperatures between -90.degree. C. and -196.degree. C. A typical dewar includes an inner tank for retaining the cryogenic fluid, such as liquid nitrogen, and biological specimens and an outer wall surrounding and spaced from the inner tank. The space between the tank and the outer wall is packed with insulating material and a vacuum is created therein A removable foam insulated cover provides access to the interior of the dewar.
To maintain the desired low temperature, a specific level of cryogenic liquid must be maintained in the dewar. Over time, however, heat transfer between the external environment and the interior of the dewar will vaporize the liquid thereby lowering the level of cryogenic liquid in the dewar. If the drop in the level of the cryogen was not corrected, the temperature in the dewar would rise to an undesirable level.
Accordingly, automatic cryogenic liquid refill systems have been developed. One such system includes a vertically extending tube in fluid flow communication with the inner tank of the dewar Cryogen will fill the tube to the same level as the cryogen in the inner tank. Thermostats for sensing the presence or absence of the cryogenic liquid are suspended in the tube on a flexible member Typically, four thermistors are used--a low sensor, a high sensor and two alarm sensors The low sensor generates a signal when the liquid cryogen reaches a predetermined minimum level to open a valve to deliver more cryogen to the dewar. The high sensor delivers a signal to shut the valve and terminate delivery of liquid cryogen when the liquid reaches a predetermined maximum level. The other two sensors are used as alarms to generate a signal should either of the high or low sensors fail. The flexible member on which the thermistors are suspended can be raised or lowered in the tube to adjust the level of liquid cryogen.
One problem with this design is that it is difficult to accurately manually adjust the location of the sensors to change the cryogenic liquid level. Another problem with such a system is that because there is no fluid flow in the tube, the extremely cold cryogen will cause ice to build up in the tube thereby preventing the operation of the thermistors. Moreover, the ice can either prevent movement of the sensors or can damage the sensors when they are moved.
In an effort to solve some of these problems, a system using a pressure transducer located at the end of the tube has been developed. A column of compressed cryogen vapor transmits changes in the level of the cryogen fluid in the dewar to the pressure transducer A solenoid valve is opened and closed in response to signals from the pressure transducer to control delivery of cryogen liquid to the dewar.
While the pressure transducer allows more accurate measurement of changes in the cryogen level, it does not provide fluid flow in the tube so that the icing problems discussed above are still present. When icing occurs in the tube, the pressure transducer's responsiveness is affected resulting in undesirable fluctuations in the liquid level in the dewar.
Thus, an improved level sensor assembly for a cryogenic dewar is desired